Many waters contain alkaline earth metal cations, such as barium, strontium, calcium and magnesium, and anions, such as sulfate, bicarbonate, carbonate, phosphate and fluoride. When combinations of these anions and cations are present in concentrations which exceed the solubility product of the various species which may be formed, precipitates form until the respective solubility products are no longer exceeded. For example, when the concentrations of the barium and sulfate ions exceed the solubility product of barium sulfate, a solid phase of barium sulfate will form as a precipitate. Solubility products are exceeded for various reasons, such as evaporation of the water phase, change the pH, pressure or temperature and the introduction of additional ions which can form insoluble compounds with the ions already present in the solution.
As these reaction products precipitate on the surfaces of the water-carrying or water-containing system, they form adherent deposits or scale. Scale may prevent effective heat transfer, interfere with fluid flow, facilitate corrosive processes, or harbor bacteria. Scale is an expense problem in any industrial water systems, in production systems for oil and gas, in pulp and paper mill systems, and in other systems, causing delays and shutdowns for cleaning and removal.
In the co-pending applications reference above, there is disclosed a method for removing barium sulfate and other sulfate scales by a solvent comprising a combination of a chelating agent comprising a catalyst or synergist comprising polyaminopolycarboxylic acid such as EDTA or DTPA together with anions of (1) a monocarboxylic acid such as acetic acid, hydroxyacetic acid, mercaptoacetic acid or salicylic acid; (2) oxalates, or (3) thiosulfates. The scale is removed under alkaline conditions, preferably at pH values of at least 10, usually 10-14, with best results being achieved at about pH 12. When the solvent becomes saturated with scale metal cations, the spent solvent is disposed of by re-injection into the subsurface formation. However, this is an expensive way of disposal and requires a fresh supply of moderately expensive solvent.
This invention provides a method to chemically remove the dissolved scale from the spent solvent through precipitation of the dissolved metal ions.